Traveling-wave tube



Sept 23, 1958 c. K. BIRDSALL ET AL 2,853,642

TRAVELING-WAVE TUBE Filed Feb. 2s, 1955 2 Sheets-Sheet l Nn Nm.

sept. 23, 195s Flled Feb 25, 1955 .f M. rf. 2 Il, .h u n mu e 0/- e, /W 4 w W iw 7 e 2 m MM 2 u n i l L AE l TB J Ew LE LW.. Dm. mm Bw K m C United States Pate-nti() 2,853,642 TRAvELING-WAVE TUBE f Application February 23, 1955, Serial No. 490,088

s Claims. (cl. sis-3.5)

This invention relates to microwave tubes and more particularly to the slow-wave structureof a'traveling wave tube. y

Recent developments in high power traveling-wave tube construction involve the use of slotted, tubular, conductive slow-wave structures, one ,ofwhichgis disclosed in a copending application, Serial No. 450,987, tiled August 19, 1954, by Charles K. Birdsall. Another copending application, Serial No. 456,682, filed September 17, 1954, of which the present application is a continuation-inpart, is directed to related slow-wave structures. These types of slow-wave structures have been employed in traveling-wave tube amplifiers to avoid self-oscillations due to backward wave type interaction within a band of frequencies normally constituted of centimeter wavelengths. A tubular slow-wave structure is thus employed to propagate anelectromagnetic wave vin the usual mau` ner at a velocity substantially less than the velocity of light whereby an electron stream may be projected coaxially through the slowwave structure at approximately the same velocity as that of-the wave. action of the stream and the wave is consequently produced, the interaction causing the wave to grow or to be amplified. A complete collimation of the stream electrons is desirable to effectV optimum interaction of the stream with the propagated wave; however,'as many as one-tenth of the stream electrons generally collide with the slow-wave structure in a high power tube, such as that disclosed in the applications above referred to, and a convection current as large as one-ampere may be produced in the slow-wave structure. The energy of the stream electrons impinging upon the slow-wave structure is turned into heat which must be removed. It is desirable to provide means to conduct this heat away from the slow-wave structure and which means does not change the slow-wave structure impedance over the oper# ating frequency band. A,

Further, when a slow-Wave structure constituted 'ofa conventional conductive helix is employed, such a helix being desirable for .low-power applications, backward wave self-oscillations may .prohibit theoperation of a traveling-wave tube as an amplier within the backward wave self-oscillation band of frequencies.

`An object of the invention is therefore to provide means for*Y conducting heat from a traveling-Wave tube slowwave structure without producing a substantial change in they impedance of that structure over the operating frequency band of the tube.

Another object ofthe invention is to provide `means for substantially suppressing backward-wave'foscillations along a traveling-wave tube slowh-wave'structurewithout producing a 'substantial change inpthe Vimpedance of that structure over the operating frequency bandof the'ftube.

*lt-fis affuther objectvof the inventionjtoprovide means for modifying the phase velocity lfrequency characteristic 4vof the slow-wave structurefbf a traveling-Wave tube to Mutual inter# '2,853,642' Patented Sept. 23, 1958 control the bandwidth of operation and, at the same time, to. increase'the impedance of the structure.

By practicing the present invention a solution to the heat conduction problem in high-power tubes or a solution to the backward wave self-oscillation problem accompanying the use of a conductive helix may be obtained as well'as the control' over the bandwidth of operation. To this end, in accordance with the invention a plurality of relatively thin axially spaced conductors are connected radially from a slow-wave structure to a hollow, cylindrical, conductive envelope, the envelope being disposed concentrically aboutl the slow-wave structure.

The novel features which are believed to be characteristic of the invention are set forth with particularity in the appended claims. The invention itself, however, both as toits organization and method of operation, togetherwith additional objects and advantages of the invention, may be better understood when taken in connection with the'accompanying drawingsin which several embodiments of the invention are illustrated by way of example. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only, and are not intended as a definition of the limits of the invention.

Fig. 1 is a sectional View of a traveling-wave amplifier embodying the present invention;

Figs. 2 and 3 are sections along the lines 2 2 and 3 3, respectively, of Fig. 1; f p

Fig. 4 is a broken orthogonal view of the heat conducting and slow-wave structureof the amplifier of Fig. l;

Fig.l5 is a broken orthogonalview of an alternative embodiment of the heat conducting and slow-wave structure of the amplifier of Fig. l;

Figs. 6 and 7k are broken sectional views of another embodiment of the slow-wave'structure of the present invention;

Figs. 8 and`9 are orthogonal broken-away views of still other embodiments of the heat conducting and slow-wave structure of the amplifier of Fig. l; and

Fig. 10 is a broken plan view of a conventional slowwave structure or conductive helix having periodic dis# continuities provided along its length kto provide the heat conducting structures of thepresent invention.

Referring to Fig. 1, a-trayeling-wave tube amplifier 10 is illustrated having a cylindrical, conductive, nonmagnetic envelope 12 which may be made of copper. An electron gun Y14 is sealed in the left extremity of the envelope, as viewedlin Fig. 1., Electron gun 14 is employed toy produce a stream of electrons and to direct it along the longitudinal axis of envelope 12.

A solenoid 16 is disposed concentrically about envelope 12 to provide an axial magnetic field along the electron stream pathwhereby the Stream may be constrained alongthe complete length ofthe envelope. Such a held may be of the order of 600 to 1200 gauss. In order to produce the magnetic field, a direct current is maintained in solenoid 16 bymeans ofV a potential source 18.

A non-magnetic conductive cooling tank 19 having the shape of adouble-walled hollow'cylinder and'which may also be made. of copperI is disposed concentrically between solenoiditi and envelope 1.2.A A liquid or gaseous coolant may thus be circulated through' the 'tank to :corr duct heat away from both solenoid 16` and envelope 12.'

Electron gun ,14 essentiallycomprises,a cathode cylinder 20,'a heater 22, a focusing electrode 24, and an accelerating anode 26. Heater 22 is connected across a suit-f able source of potential `28, the negative side of Vtheheater beingconnected to cathode, 20. The negative side of source 28` is then connected to. the .negative terminal of airpotential; source 30,1 the positive'side of whichis ,connected 'to'ground in forder to maintain cathode 20 at .3 u a potential of about 30,000 to 35,000 volts negative with respect to ground. Focusing electrode 24 has a frustoconical shape with an internal surface of revolution forming an angle of 671/2 degrees with its axis of symmetry. Focusing electrode 24 is maintained at the same potential as cathode 20 by an vappropriate connection thereto. Accelerating anode 26 ismaintained at about 200 volts positive with respect to ground by a `connectionto a tap 32 on potential source 30.

A disc-shaped magnetic pole piece 33 and dielectric Aspacer 34 are disposed contiguously to a cylindrical appendage 27 of anode-*26 to maintain the gun 14 in alignment with envelope 12. A glass stem 36 with integral leads 37 welded to the gun 14 and an anode lead 38 comprise the remaining supports of gun 14.

Within envelope 12, adjacent to and to the right of gun 14, as viewed in Fig. 1, slow-wave structure 40 is provided in electrical contact with and supported in part by a rectangular internal input waveguide segment 42 and a rectangular internal output waveguide segment 44. The slow-wave structure 40 and waveguide segments 42 and 44, which may consist of copper, are all maintained at ground potential by an electrical connection 4S from envelope 12 to ground. As will be evident from Figs. 4, 5, 6, 8, 9, and 10, the slow-wave structure 40 is the electrical equivalent of at least one helical conductor. Waveguide segments 42 and 44 are positioned in contact with envelope 12. This will be better understood as hereinafter explained in connection with Figs. 2 and 3.

Slow-wave structure 40 comprises a plurality of coaxial conductive rings 41, one set of adjacent pairs of rings being connected at one arcuate portion 57 and the alternate set of pairs being connected to a portion 58 diametrically opposite portion 57. Waveguide segments 42- and 44 have transverse end portions 43 and 54, respectively, which have facing apertures 53 and S5 respectively, for launching a traveling-wave and for decoupling the wave at the output end of the slow-Wave structure 40.

An external input waveguide segment 59 and an external output waveguide segment 60 are supported by suitable apertures in a disc-shaped dielectric spacer 49 which is disposed concentrically within the right end of -envelope 12 adjacent a magnetic pole piece 180. Each of the external waveguide segments 59 and 60 are provided at their outer ends with mica windows 62'and 63, respectively, which provide vacuumk seals. A'vacuum is thusmaintained from the windows 62 and' 63 to the opposite end of envelope 12 at the extreme left of glass stem 36.l

The streamelectrons are intercepted by ,a collector electrode 52 atthe opposite extremity of envelope 12 with respect to electron gun 14. Collector 52, which protrudes outside of the envelope 12, is also supported by a suitable aperture in dielectric spacer 49 so as to have a large surface external to the evacuated chamber for heat dissipation purposes and may include fins as shown to aid in conducting away the heat that is generated by the stream electrons when they are collected. Accordingly, collector 52 is preferably fabricated of a metal having good electrical and heat conducting properties, as in the case of structure 40, such as, for example, copper. A potential of the order of 200 volts positive withrespect to structure 40 -is applied to collector 52 inorder to prevent secondary. electrons which may be produced by the stream electrons impinging upon its surface from reaching slow-wave structure 40. This potential is applied by means of a connection from collector 52 to lthepositive terminal of a source 56, the negative terminal of which is grounded.

A plurality of conductive ns 51- are connected electricallyand mechanically lengthwise of and from slow-wave structure 40 to a conductive cylinder -64 disposed within envelope -12. The conductive cylinderh64-is thenfmechanically and electrically-connected to v.internal Ainput waveguide segment 42 in order that heat may be conducted from slow-wave structure through.conductive tins 51, conductive cylinder 64 and internal input waveguide segment 42 to envelope 12. This is better illustrated in Figs. 2 and 3 where solenoid 16, tank 19 and envelope 12 are respectively shown disposed concentric with one another. Internal input waveguide segment 42 isshown to be in contact with envelope 12 at two edges 66 and 68 in both Figs.` 2 and 3 and in contact with conductive cylinder 64 at a point 70. In Fig. 2 a vertical4 fin 51 is shown connected to conductive cylinder 64-and to anaxial conductor 72 of the conductive portion 57 connecting an adjacent pair of the conductive rings 41. In Fig. 3 two vertical iins 51, 51 are shown connected between a particular conductive ring 78 of the conductive rings 41 and conductive cylinder 64.

Fins 51 are shown to be in the same angular position in Fig. l although it is their axial positions or periodicity with respect to slow-wave structure 40 that makes them useful and not their particular arcuate positions. Each of the ns 51 has an arcuate thickness small in comparison to the external circumference of slow-Wave structure-40 and extends axially over two of the cylinders 41 as clearly shown in Fig. l. Each of the ns extends radially from the slow-wave structure 40 a distance` equal-to T where n is any positive odd integer and ko is the free space wavelength corresponding to the mid-frequency of the operating band for which the tube 10 is designed. This requirement isfmade in order to'avoid changing the impedance of the slow-wave structure in the center of of the slow-wave structure 40 would then be lost.

the operatingn band, and very little about the` center. When the iins 51 extend the above specified distance from slow-wave structure 40, a virtual shorting plane preduced by conductive cylinder 64 will be prevented from appearing at the periphery of the slow-wave structure 40 between adjacent rings 41 to short electromagnetic energy therebetween. Wave propagation at the bandV center will thus be essentially the same with or without the use of fins 51.

The operation of the traveling-wave tube 10 is substantially the same as that explained in the previously mentioned sole application of Birdsall. Thus in the device explained in that application and in the traveling-wave tube of the present invention an input signal to be amplied is impressed upon the input waveguide. A traveling wave is then launched on the slow-wave structure permitting interaction of the traveling wave with the electron stream projected from the electron gun. This interaction results in a transfer of energy from the electron stream to the wave causing the wave to grow or increase in amplitude as it progresses along the slow-wave structure. The amplified wave energy is then coupled from the output waveguide segment to a utilization device.

Fig. 4 is a broken orthogonal view of slow-wave structure 40, tins 51 and conductive cylinder 64. Fins 51 are shownwith a maximum axial length in order to provide a maximum heat conducting cross section. Fins 51 can be made much shorter axially, but they can be made no longer because the desirable wave propagation properties The slow-wave structure 40 is thus seen to be the electrical equivalent of at least one helical conductor and in fact as illustrated in Fig. 4 is deemed to simulate uniilar contrawound helices having a finite pitch equal to a dimension 140. This is described in detail in the sole application of Birdsall referred to above. It is well known that periodic discontinuities along a waveguide structure, such as the tins 51 along the slow-wave structure 40, provide stop bands when they occur at a spacing or interval, L, equal to where Ag is the unloaded waveguide wavelength of the stop band mid-frequency and m is any positive integer. It is obvious that there is an upper and lower n 51 dis posed along the slow-wave structure 40 per pitch length in Fig. 4. Heat conduction nevertheless may be provided with tins on only one side of the slow-wave structure 40. This is illustrated in Fig. 5.

An alternative embodiment of the slow-wave structure of the present invention is shown in Figs. 6 and 7 where alternate -pairs of adjacent conductive rings 41 are connected at diametrically opposed points by a plurality of conductive sheets 200 which extend tangentially from the rings 41 to conductive cylinder 64 in parallel planes. By the conductive sheets 200, heat may be eiiiciently conducted away from the conductive rings 41.

An additional embodiment of the slow-wave structure of the present invention is shown in Fig. 8 comprising a serpentine conductive strip 300 folded upon itself and having a plurality of registering apertures 302 therein to permit passage of an electron stream therethrough. A plurality of conductive columns or stubs 303 'are then connected from the strip 300 to a conductive cylinder 304 which is positioned about the stubs 303 for heat conduction purposes. The slow-wave structure of Fig. 8 is electrically equivalent to that of Fig. 4 and hence the electrical equivalent of at least one conductive helix, the principal change in structure being in the angular width of the axially conductive segments 57, 58 and conductive fins 51. Planar transverse segments-306 of the conductive strip 300 are equivalent to conductive rings 41 and conductive iins 51 are represented by the plurality of radially conductive stubs 303.

In Fig. 9 a somewhat diierent slow-wave structure 142 is shown with conductive fins 144 connected therefrom to a broken-away conductive cylinder 146. The slow wave structure 142 is provided with straight axial segments 148 and 149 which connect alternate adjacent pairs of registering conductive rings 150 at diametrically opposed points. The axially straight conductive segments 148 are then positioned 90 degrees apart from the axially straight conductive segments 149. The slow-wave structure 142 thus provides a path which is electrically equivalent to at least one helical conductor and as shown simulatesbilar contrawound helices having a pitch indicated by dimension 152. This is likewise described in the sole application of Birdsall referred to above. As in the case of the loading of the slow-wave structure 40 in Fig. 4 slow-wave structure 142 in Fig. 9 is loaded twice per pitch. If the nature of the slowwave structure 142 is closely considered, it is obvious that if one or more of the axially straight conductive segments 148 and 149 have conductive fins 144 extending therefrom, the simulated contrawound helices of the slow-wave structure 142 may be loaded four, six, or eight times per pitch length.

In Fig. 10 a conventional type of conductive helix 170 for a traveling-wave tube is illustrated having a plurality of radially conductive ns 172 extending in one direction only and which have their radial extremitiesv connected by a conductive cylinder 174. The helix 17 0 may be constructed in the form of a tape or wire, although a tape may have better heat dissipation capabilities.

In traveling-wave tubes employing unitilar and multitilar conductive helices, backward wave self-oscillations are troublesome. About the backward wave self-oscillation band, the practical operation of a traveling-wave tube solely as an amplifier is impossible. Conductive tins 172 may lthus be employed to achieve two objectives. Firstly, conductive fins 172 may, as in the case of conductive iins Sil, be used to conduct heat from helix 170 to conductive cylinder 174. Secondly, conductive fins 172 may in addition be employed to provide a stop band at the backward wave self-oscillation band of frequencies for the helix 170. The angular alignment of the conductive iins shown in Fig. 10 is, of course, unnecessary although the relationship should be maintained, where d is the axial spacing of the conductive iins 172 and xg is the unloaded guide wavelength at the mid-frequency of the backward wave selfoscillation band. The yradial length of the conductive ns 172 may be in accordance with the rule of design set down with respect to the radialA length of the conductive fins 51 shown in Fig. l. The axial length of conductive fins l172 may be equal to the width of the tape forming the helix 170. This isanalogous to the case of the axial length of conductive ns 51 which have an axial dimension equal to the axially conductive lengthof the specied portions of the slow-wave structure 40 to which ns 51 are connected, i. e., equal tothe sum of the width of two of the conductive rings 41 andthe length of an ially conductive segment connecting two adjacent rings By the use of any of the embodiments of the device of the present invention stop bands may be provided or avoided as desired and heat may be conducted from traveling-wave tube slow-wave structures without reducing their forward wave impedances or their impedances to waves within their respective operating frequency bands.

What is claimed is:

1. A wave-type amplifier comprising an evacuated en velope, an electron gun disposed at one end of said envelope'for producing an electron stream, a collector electrode disposed at the opposite end of said envelope to lnterceptthestream electrons, means for directing said electron stream along a predetermined path from said electron gun to said collectorelectrode, a conductive slow-wave structure for :propagating electromagnetic waves within a predetermined frequency band having a mld-frequency corresponding toV a .free space wavelength of A0, said slow-wave structureincluding a plurality of axially spaced conductive rings disposed concentrically about said path, and means electrically interconnecting adjacent rings alternately at diametrically opposed points; a plurality of conduct-ive ns, each of said conductive ns extending axially over two adjacent conductive rings in electrical contact therewith at the arcuate position of at least one of said points, said conductive lins extending radially from said conductive rings a distance substantially equal to wheren is a positive odd integer, and a conductive cylinder disposed about said conductive iins, the outer extremltles of said conductive ns being in physical and electrical contact with said conductive cylinder, whereby v heat may be conducted away from said slow-wave structure without materially reducing the impedance of said slow-wave structure to waves of frequencies within said predetermined band.

2. A wave-type amplifier comprising an evacuated envelope; an electron gun disposed at one end of said envelope for producing an electron stream; a collector electrode disposed at the opposite end of said envelope to intercept the stream electrons; means for directing said stream along a predetermined path from said electron gun to said collector electrode; a slow-wave structure disposed contiguously about said path for propagating waves within a predetermined frequency band having a mid-frequency corresponding to a free space wavelength of A0, said slow-wave structure including a plurality of axially spaced conductive cylinders disposed concentrically about said path, and means electrically interconnecting adjacent cylinders at a pair of. diametrically opposed points, consecutive-pairs of said diametrically opposed points being displaced 90 angular degrees with respectgto each other about said path; a plurality of conductivefiins, each of said conductive fins extending axially over two adjacent conductivey cylinders in electrical contact V,therewith at the arcuate-position of atleast one of said points, said conductive fins extending radially vfrom said conductive cylinders a distance substantially equal to where n` is `any Vpositive odd integer; and conductive means connecting the outerrextremities of said conductive fins, -whereby heatV may be conducted away from said slow-wave structure without materially, reducing the impedance .ofsaid 'slow-Wave structure towaves Vof frequencies within said predetermined band.

3. In a traveling-wave tube, anY integral slow-wave structure and slowawave 'structure support for propagating electromagnetic waves within a'predetermined frequency band having amid=frequency corresponding to a free space wavelength of A comprising a serpentine conductive strip including aj plurality of parallel transverse segments having registeringapertures Vand a plurality of axially conductive' 'segments' connecting adjacent .pairs of said transverse segments alternately at their opposite ends, a conductive cylinder spaced from`the 'slow-wave structure, where n vis any positive odd integer and A0 is the free space wavelength of the mid-frequency -of theV operating band of said amplifier, anda plurality of conductive stubs connected radially from said axially conductive segments to said conductive cylinder having an' effective' length substantially equal to where n is any positive integer.

4. A wave-type amplifier comprising an evacuated envelope, an electron gun'disposed at one end of saidl envelope for producing an electron stream, a collector electrode disposed at the opposite end of said envelope to intercept the stream electrons, means for directing said electron stream along a predetermined path from said electron gun to said collector electrode, a conductive slow-wave structure for propagating electromagnetic waves within a predetermined frequency band having a mid-frequency corresponding to a free space wavelength of A0, said slow-wave structure including a plurality of axially spaced conductive rings disposed concentrically about said path, and means electrically interconnecting adjacent rings alternately at diametrically opposed points; a plurality of conductive tins having an arcuate thickness small in comparison to the circumference of said slowwavestructure, each of said conductive fins extending axially over' two adjacent conductive rings in electrical contact therewith at the arcuate position of each one of saidpoints, said conductive ns extending radially from said conductive rings a distance substantially equal to where 4n is a positive odd integer, and a conductive cylinder disposed about said conductive ns, the outer extremites of said. conductive fins being in` physical and electrical contact with said conductive cylinder, whereby heat may be conducted away from said slow-wave structure Without materially `reducing the impedance of said slowwave. structurek towaves of frequencies within said predetermined band.V

5. .A wave-type amplifier comprising an evacuated envelope; an electron gun disposed at one end of. said envelope for producing an electron stream; a collector electrodedisposed at the opposite. end of said envelope to intercept the stream electrons; means for directing said stream along a predetermined path from said electron gun to said collector electrode; a slow-wave structure disposed contiguouslyabout said path for propagating waves .within a predetermined frequency band having a mid-frequency corresponding to a free space wavelength of 1M, .said slow-wave structure including a plurality of axially spaced conductive cylinders disposed concentricallyk aboutk said path, and meansV electrically interconnecting'adjacent cylinders at a pair of diametrically opposed points, consecutive pairs of said diametrically opposed points beingdisplaced angular degrees with respect to each other about said path; a plurality of conductivens having an arcuate thickness small in comparison to the circumference of saidA slow-wave structure, each of said conductive tins extending axially over two adjacent conductive cylinders in electrical contact therewithat the arcuate position of each of said points, said conductive fins extending radially from said conductive cylinders a distance substantially equal to ReferencesvCited in the le of this patent UNITEDv STATES PATENTS Pierce Apr. 28, 1953 2,683,256 Kumpfer July 6, 1954 2,794,144 White May.28, 1957 2,794,145 Bryant May 28, 1957 A Patent Noo 25,853,642

UNITED STATES PATENT OFFICE l CERTIFICATE OR 4CORRECTION A l September 239 1958 'Charles K; RirdSaII @i @LIQ` It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Pateht should read as corrected below.

Column 8 line 47, for an positive" read m any positive wo Signed and sealed this 27th day of January 195% SEAL (Attes: l KARL R AXLINR ROBERT C. WATSON Commissioner of Patents Attesting Officer 

